Ø Ordinary
Thunderstorms
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Define: Isolated thunderstorms that form in the
absence of strong vertical wind shear.
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Often form along weak boundaries, such as cool
air outflows from other thunderstorms
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Tend to form in hot afternoons
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Difference between environments for ordinary
thunderstorms and severe is the strength of the winds aloft.
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Winds aloft are typically weak and the vertical
wind shear minimal
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Far from frontal boundaries
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Three
stages of growth:
1.
Cumulus
stage: cloud consists of a warm, buoyant plume of rising air -
the updraft (increases rapidly with height)
2.
Mature
stage: can be identified visually by examining the upper part of
the storm. Air from the updraft exhausts horizontally at the tropopause forming
the anvil, Mammatus clouds often form on the bottom of the anvil. Precipitation
forming in the upper part has to fall vertically with through the updraft.
3.
Dissipation
stage: typically produce and outflow of cool air. Clouds (shelf
or roll) will typically form over the gust front as warm air is lifted over the
spreading cold pool. As the cool air rushes out more thunderstorms cells may
trigger near the cool air’s leading edge as the warm air is lifted.
Ø Mesoscale
Convective Systems
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Produce much of the summer rainfall on the
Central Plains of North America
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Damaging straight line winds and occasional
weak tornados
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Lifting of air along weak airmass boundaries
often will trigger thunderstorms, particularly in the late afternoon.
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As the storm progresses, the storms will begin
to organize often becoming more intense and progressively aligning into an
arc-shaped line (Squall Line)
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One or more segments along the squall line may
bow outward, producing a bow echo on radar. During this time, a region of
widespread less-intense precipitation develops to the rear of the squall line.
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Eventually, the thunderstorms decay, leaving
their wake a wide stratiform region that itself may continue to produce rain
for several more hours. During this time, new thunderstorms may develop.
Ø Frontal
Squall Lines
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Form in the warm, moist air ahead of surface
cold fronts and dry lines, or just ahead of the upper-level front.
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Typically hundreds of kilometers long
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Commonly form the “tail” of the comma-cloud
pattern in extratropical cyclones, particularly those that form over land.
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Long lifelines, lasting many hours or days
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Can produce weak tornados and hail but tend to
be most associated with strong straight-line winds.
Ø Supercell
Thunderstorms
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A supercell is a storm with a mesocylone
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Most intense thunderstorms in Earth’s
atmosphere.
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Always rotate (entire storm behaves as a single
entity, rather than as a group of cells. example: MSC)
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Produces virtually most all severe tornados,
large hail (can grow as large as grapefruits), and damaging straight-line winds
(typically 20-40 meters/second or 45-90 mph)
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Four key ingredients to form:
1.
An environment that is conditionally unstable
(humid and warm at the surface and dry and very cold aloft)
2.
Very moist air in lower troposphere.
3.
Moderate to strong wind shear (through the
depth of the unstable layer)
4.
A trigger mechanism (lifting along a boundary)
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The larger the CAPE the stronger a
thunderstorms updraft may become (all else being equal)
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It is critical that the environment winds
increase rapidly with height within the layer of maximum instability.
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Supercells have a tendency to first erupt where
boundary intersections occur (such as, the intersection of the dry line and
warm front)
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New cells often trigger in rapid succession
along the boundaries, creating a line of supercells.
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Precipitation rarely appears in the core of the
updraft region, leaving a rain-free base.
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Tornados within classic supercells nearly
always occur outside of the heavy precipitation region of the storm because
they occur in the vicinity of the intense updraft.
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Typically occur where strong middle and upper
tropospheric jet streams are present.
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Inversion: Temperature
is warmer outside the cloud, not unstable
o
How do I know there is an inversion? Bob
o
Lifted Index (LI): measure of instability
